This contractor acts as a clinical center collecting patient data for assessment of performance of ultrasound B-scan systems. The add-on portion of the contract requires the contractor to collect follow-up data for approximately 400 patients with the objective of determining how well the procedure of B-scanning can detect progression or regression of lesions in the non-operable carotid artery of patients who have had clinically significant lesions in the other carotid artery. All data will be sent to the Data Coordinating Center for analysis.